Evidence for neurotoxicity and oxidative stress in zebrafish embryos/larvae treated with HFPO-DA ammonium salt (GenX).

"GenX" [ammonium perfluoro (2-methyl-3-oxahexanoate] was developed as a replacement chemical for toxic perfluorinated compounds to be used in product manufacturing. Here, we assessed developmental, mitochondrial, and behavioral toxicity endpoints in zebrafish embryos/larvae exposed to GenX. GenX exerted low toxicity to zebrafish embryos/larvae up to 20 mg/L. GenX did not affect mitochondrial oxidative phosphorylation nor ATP levels. ROS levels were reduced in larvae fish exposed to 10 and 100 µg/L, indicative of an antioxidant defense; however, ROS levels were elevated in fish exposed to 1000 µg/L. Increased expression of cox1 and sod2 in GenX exposed 7-day larvae was noted. GenX (0.1 or 1 µg/L) altered transcripts associated with neurotoxicity (elavl3, gfap, gap43, manf, and tubb). Locomotor activity of larvae was reduced by 100 µg/L GenX, but only in light periods. Perturbations of anxiety-related behaviors in larvae were not observed with GenX exposure. These data inform risk assessments for long-lived perfluorinated chemicals of concern.

Effects of the ammonium stress on photosynthesis and ammonium assimilation in submerged leaves of Ottelia cordata - an endangered aquatic plant.

Although ammonium (NH4+-N) is an important nutrient for plants, increases in soil nitrogen (N) input and atmospheric deposition have made ammonium toxicity a serious ecological problem. In this study, we explored the effects of NH4+-N stress on the ultrastructure, photosynthesis, and NH4+-N assimilation of Ottelia cordata (Wallich) Dandy, an endangered heteroblastic plant native to China. Results showed that 15 and 50 mg L-1 NH4+-N damaged leaf ultrastructure and decreased the values of maximal quantum yield (Fv/Fm), maximal fluorescence (Fm), and relative electron transport rate (rETR) in the submerged leaves of O. cordata. Furthermore, when NH4+-N was ≥ 2 mg L-1, phosphoenolpyruvate carboxylase activity (PEPC) and soluble sugar and starch contents decreased significantly. The content of dissolved oxygen in the culture water also decreased significantly. The activity of the NH4+-N assimilation enzyme glutamine synthetase (GS) significantly increased when NH4+-N was ≥ 10 mg L-1 and NADH-glutamate synthase (NADH-GOGAT) and Fd-glutamate synthase (Fd-GOGAT) increased when NH4+-N was at 50 mg L-1. However, the activity of nicotinamide adenine dinucleotide-dependent glutamate dehydrogenase (NADH-GDH) and nicotinamide adenine dinucleotide phosphate-dependent glutamate dehydrogenase (NADPH-GDH) did not change, indicating that GS/GOGAT cycle may play an important role in NH4+-N assimilation in the submerged leaves of O. cordata. These results show that short-term exposure to a high concentration of NH4+-N is toxic to O. cordata.

Assessment of antibacterial properties and skin irritation potential of anodized aluminum impregnated with various quaternary ammonium.

The importance of the inert environment in the transmission of pathogens has been reassessed in recent years. To reduce cross-contamination, new biocidal materials used in high touch surfaces (e.g., stair railings, door handles) have been developed. However, their impact on skin remains poorly described. The present study aimed to evaluate the antibacterial properties and the risk of skin irritation of two materials based on hard-anodized aluminum (AA) impregnated with quaternary ammonium compound solutions (QAC#1 or QAC#2). The QAC#1 or QAC#2 solutions vary in composition, QAC#2 being free of dioctyl dimethyl ammonium chloride (Dio-DAC) and octyl decyl dimethyl ammonium chloride (ODDAC). Unlike AA used as a control, both AA-QAC#1 and AA-QAC#2 had excellent and rapid antibacterial efficacy, killing 99.9 % of Staphylococcus aureus and Escherichia coli bacteria, in 15 s and 1 min, respectively. The impregnation solutions (QAC#1 and QAC#2) did not show any skin sensitizing effect on transformed human keratinocytes. Nevertheless, these solutions as well as the materials (AA-QAC#1, AA-QAC#2), and the liquid extracts derived from them, induced a very rapid cytotoxicity on L929 murine fibroblasts (>70 % after 1 h of contact) as shown by LDH, MTS and neutral red assays. This cytotoxicity can be explained by the fast QACs release occurring when AA-QAC#1 and AA-QAC#2 were immersed in aqueous medium. To overcome the limitation of assays based on liquid condition, an in vitro skin irritation assay on reconstructed human epidermis (RHE) was developed. The effect of the materials upon their direct contact with the epidermis grown at the liquid-air interface was determined by evaluating tissue viability and quantifying interleukin-1 alpha (IL-1α) which is released in skin during injury or infection. AA-QAC#1 induced a significant decrease in RHE viability, close to OECD and ISO 10993-10 acceptability thresholds and enhanced the pro-inflammatory IL-1α secretion compared with AA-QAC#2. Finally, these results were corroborated by in vivo assays on mice using erythema and edema visual scores, histological observations, and epidermal thickness measurement. AA had no effect on the skin, while a stronger irritation was induced by AA-QAC#1 compared with AA-QAC#2. Hence, these materials were classified as moderate and slight irritants, respectively. In summary, this study revealed that AA-QAC#2 without Dio-DAC and ODDAC could be a great candidate for high touch surface applications, showing an extremely effective and rapid bactericidal activity, without inducing adverse effects for skin tissue.

The alleviation of ammonium toxicity in plants.

Nitrogen (N) is an essential macronutrient for plants and profoundly affects crop yields and qualities. Ammonium (NH4 + ) and nitrate (NO3 - ) are major inorganic N forms absorbed by plants from the surrounding environments. Intriguingly, NH4 + is usually toxic to plants when it serves as the sole or dominant N source. It is thus important for plants to coordinate the utilization of NH4 + and the alleviation of NH4 + toxicity. To fully decipher the molecular mechanisms underlying how plants minimize NH4 + toxicity may broadly benefit agricultural practice. In the current minireview, we attempt to discuss recent discoveries in the strategies for mitigating NH4 + toxicity in plants, which may provide potential solutions for improving the nitrogen use efficiency (NUE) and stress adaptions in crops.

Soil ammonium (NH4+) toxicity thresholds for restoration grass species.

Ionic rare earth mining has resulted in large amounts of bare soils, and revegetation success plays an important role in mine site rehabilitation and environmental management. However, the mining soils still maintain high NH4+ concentrations that inhibit plant growth and NH4+ toxicity thresholds for restoration plants have not been established. Here we investigated the NH4+ toxicological effects and provided toxicity thresholds for grasses (Lolium perenne L. and Medicago sativa L.) commonly used in restoration. The results show that high NH4+ concentration not only reduces the plant biomass and soluble sugars in leaves but also increases the H2O2 and MDA content, and SOD, POD, and GPX activities in roots. The SOD activities and root biomass can be adopted as the most NH4+ sensitive biomarkers. Six ecotoxicological endpoints (root biomass, soluble sugars, proline, H2O2, MDA, and GSH) of ryegrass, eight ecotoxicological endpoints (root biomass, soluble sugars, proline, MDA, SOD, POD, GPX, and GSH) of alfalfa were selected to determine the threshold concentrations. The toxicity thresholds of NH4+ concentrations were proposed as 171.9 (EC5), 207.8 (EC10), 286.6 (EC25), 382.3 (EC50) mg kg-1 for ryegrass and 171.9 (EC5), 193.2 (EC10), 234.7 (EC25), 289.6 (EC50) mg kg-1 for alfalfa. The toxicity thresholds and the relation between plant physiological indicators and NH4+ concentrations can be used to assess the suitability of the investigated plants for ecological restoration strategies.

Use of Toxicity Identification Evaluation Procedures to Clarify the Relationship Between Ammonium Concentrations and Phytoplankton Blooms in the San Francisco Bay Estuary, California, USA.

Phytoplankton blooms in the northern San Francisco Bay Estuary have historically supported much of the larval fish production in the estuary. In the past, blooms were limited largely by reduced light intensities and net outflows through the system, as well as dense populations of introduced clams that continuously filter the water column. Conversely, the estuary is exposed to a wide variety of contaminants that may also impact phytoplankton growth. Interestingly, previous investigations have suggested that relatively low concentrations of ammonium may inhibit development of bloom conditions by interfering with nitrate assimilation. Given the complex dynamics of the system, with multiple factors that could potentially affect algal growth, additional data to validate this hypothesis are important to identify appropriate management options. Consequently, toxicity identification evaluation (TIE) procedures were applied to ambient water samples and monitored for 72-96 h under controlled conditions to evaluate their effects on algal growth and utilization of dissolved inorganic nitrogen. The TIE treatments specifically targeted ammonium, as well as the potential contributions of metals and nonpolar organic contaminants. Notably, all samples exhibited positive growth over the exposure period with no evidence of toxicity, and TIE treatments did not further improve growth. A subsequent 72-h study evaluated the effect of ammonium up to 12 µM at a fixed concentration of nitrate was monitored at 24-h intervals and showed no inhibition of the development of bloom conditions. Collectively, there was no evidence that ammonium interfered with growth, even at concentrations well above the range of postulated effect levels. Of additional interest, the lack of increased growth in TIE treatments targeting chelatable metals and nonpolar organics suggested that these contaminant classes were not present at inhibitory concentrations. These results demonstrate the importance of validation of cause in multistressor environments, and further clarify the roles of different factors that may limit development of bloom conditions in the estuary. Environ Toxicol Chem 2023;42:178-190. © 2022 SETAC.

Glyoxalase I activity affects Arabidopsis sensitivity to ammonium nutrition.

KEY MESSAGE: Elevated methylglyoxal levels contribute to ammonium-induced growth disorders in Arabidopsis thaliana. Methylglyoxal detoxification pathway limitation, mainly the glyoxalase I activity, leads to enhanced sensitivity of plants to ammonium nutrition. Ammonium applied to plants as the exclusive source of nitrogen often triggers multiple phenotypic effects, with severe growth inhibition being the most prominent symptom. Glycolytic flux increase, leading to overproduction of its toxic by-product methylglyoxal (MG), is one of the major metabolic consequences of long-term ammonium nutrition. This study aimed to evaluate the influence of MG metabolism on ammonium-dependent growth restriction in Arabidopsis thaliana plants. As the level of MG in plant cells is maintained by the glyoxalase (GLX) system, we analyzed MG-related metabolism in plants with a dysfunctional glyoxalase pathway. We report that MG detoxification, based on glutathione-dependent glyoxalases, is crucial for plants exposed to ammonium nutrition, and its essential role in ammonium sensitivity relays on glyoxalase I (GLXI) activity. Our results indicated that the accumulation of MG-derived advanced glycation end products significantly contributes to the incidence of ammonium toxicity symptoms. Using A. thaliana frostbite1 as a model plant that overcomes growth repression on ammonium, we have shown that its resistance to enhanced MG levels is based on increased GLXI activity and tolerance to elevated MG-derived advanced glycation end-product (MAGE) levels. Furthermore, our results show that glyoxalase pathway activity strongly affects cellular antioxidative systems. Under stress conditions, the disruption of the MG detoxification pathway limits the functioning of antioxidant defense. However, under optimal growth conditions, a defect in the MG detoxification route results in the activation of antioxidative systems.

Cytogenotoxicity of fifth-generation quaternary ammonium using three plant bioindicators.

The investigation aimed to determine the cytogenotoxic effect of fifth-generation quaternary ammonium using three plant species as bioindicators. Bulbs of A. cepa and seeds of L. culinaris and P. sativum were exposed to different concentrations of fifth-generation quaternary ammonium and a control solution of distilled water for 72 h. The results showed that the A. cepa bioindicator presented the greatest reduction in root length at 50 mg L-1 and no mitotic index at 40 and 50 mg L-1, reaching 100% mitotic inhibition. Cell abnormalities were present among the three bioindicator species, where the highest index of micronuclei occurred at 50 mg L-1, being A. cepa the bioindicator with the highest relative rate of abnormality (25.28%). It was concluded that fifth-generation quaternary ammonium, in all treatments, caused a cytogenotoxic effect on the apical meristematic cells of the three species, A. cepa was the most sensitive species.

Long-term sex-dependent inflammatory response of adult frogs to ammonium exposure during the larval stage.

Among others, the global change involves a worldwide increase in cropland area, with the concomitant rise in nitrogenous fertilizer supplementation and species range alterations, including parasites and pathogens. As most animals rely on their immune systems against these infectious agents, studying the potential effects of nitrogenous compounds on animal immune response is vital to understand their susceptibility to infections under these altered circumstances. Being subjected to an alarming process of global declines, amphibians are the object of particular attention, given their sensitivity to these compounds, especially to ammonium. Moreover, whereas adults can actively avoid polluted patches, larvae are confined within their waterbodies, thus exposed to contaminants in it. In this work, we test whether chronic exposure to a sublethal dose of ammonium during the larval stage of Pelophylax perezi frogs, released from all contamination after metamorphosis, leads to impaired inflammatory response to phytohemagglutinin in adults. We also test whether such a response differs between agrosystem individuals as compared with conspecifics from natural habitats. We found negative carryover effects of chronic exposure of larvae to ammonium on adult inflammatory response, which could imply a greater susceptibility to pathogens and parasites. However, this damage is only true for males, which, according to the immunocompetence handicap hypothesis, could be a consequence of a testosterone-triggered impairment of male immune function. In disagreement with our prediction, however, we detected no differences in the inflammatory response of agrosystem frogs to phytohemagglutinin as compared with natural habitat conspecifics.

Does energy cost constitute the primary cause of ammonium toxicity in plants?

Nitrate (NO3-) and ammonium (NH4+) are the main nitrogen (N) sources and key determinants for plant growth and development. In recent decades, NH4+, which is a double-sided N compound, has attracted considerable amounts of attention from researchers. Elucidating the mechanisms of NH4+ toxicity and exploring the means to overcome this toxicity are necessary to improve agricultural sustainability. In this review, we discuss the current knowledge concerning the energy consumption and production underlying NH4+ metabolism and toxicity in plants, such as N uptake; assimilation; cellular pH homeostasis; and functions of the plasma membrane (PM), vacuolar H+-ATPase and H+-pyrophosphatase (H+-PPase). We also discuss whether the overconsumption of energy is the primary cause of NH4+ toxicity or constitutes a fundamental strategy for plants to adapt to high-NH4+ stress. In addition, the effects of regulators on energy production and consumption and other physiological processes are listed for evaluating the possibility of high energy costs associated with NH4+ toxicity. This review is helpful for exploring the tolerance mechanisms and for developing NH4+-tolerant varieties as well as agronomic techniques to alleviate the effects of NH4+ stress in the field.

Genome-wide identification of resistance genes and transcriptome regulation in yeast to accommodate ammonium toxicity.

BACKGROUND: Ammonium is an important raw material for biomolecules and life activities, and the toxicity of ammonium is also an important ecological and agricultural issue. Ammonium toxicity in yeast has only recently been discovered, and information on its mechanism is limited. In recent years, environmental pollution caused by nitrogen-containing wastewater has been increasing. In addition, the use of yeast in bioreactors to produce nitrogen-containing compounds has been developed. Therefore, research on resistance mechanisms that allow yeast to grow under conditions of high concentrations of ammonium has become more and more important. RESULTS: To further understand the resistance mechanism of yeast to grow under high concentration of ammonium, we used NH4Cl to screen a yeast non-essential gene-deletion library. We identified 61 NH4Cl-sensitive deletion mutants from approximately 4200 mutants in the library, then 34 of them were confirmed by drop test analysis. Enrichment analysis of these 34 genes showed that biosynthesis metabolism, mitophagy, MAPK signaling, and other pathways may play important roles in NH4Cl resistance. Transcriptome analysis under NH4Cl stress revealed 451 significantly upregulated genes and 835 significantly downregulated genes. The genes are mainly enriched in: nitrogen compound metabolic process, cell wall, MAPK signaling pathway, mitophagy, and glycine, serine and threonine metabolism. CONCLUSIONS: Our results present a broad view of biological pathways involved in the response to NH4Cl stress, and thereby advance our understanding of the resistance genes and cellular transcriptional regulation under high concentration of ammonium.

Pulmonary exposure of mice to ammonium perfluoro(2-methyl-3-oxahexanoate) (GenX) suppresses the innate immune response to carbon black nanoparticles and stimulates lung cell proliferation.

BACKGROUND: Per- and polyfluoroalkyl substances (PFAS) have been associated with respiratory diseases in humans, yet the mechanisms through which PFAS cause susceptibility to inhaled agents is unknown. Herein, we investigated the effects of ammonium perfluoro(2-methyl-3-oxahexanoate) (GenX), an emerging PFAS, on the pulmonary immune response of mice to carbon black nanoparticles (CBNP). We hypothesized that pulmonary exposure to GenX would increase susceptibility to CBNP through suppression of innate immunity. METHODS: Male C57BL/6 mice were exposed to vehicle, 4 mg/kg CBNP, 10 mg/kg GenX, or CBNP and GenX by oropharyngeal aspiration. Bronchoalveolar lavage fluid (BALF) was collected at 1 and 14 days postexposure for cytokines and total protein. Lung tissue was harvested for histopathology, immunohistochemistry (Ki67 and phosphorylated (p)-STAT3), western blotting (p-STAT3 and p-NF-κB), and qRT-PCR for cytokine mRNAs. RESULTS: CBNP increased CXCL-1 and neutrophils in BALF at both time points evaluated. However, GenX/CBNP co-exposure reduced CBNP-induced CXCL-1 and neutrophils in BALF. Moreover, CXCL-1, CXCL-2 and IL-1ß mRNAs were increased by CBNP in lung tissue but reduced by GenX. Western blotting showed that CBNP induced p-NF-κB in lung tissue, while the GenX/CBNP co-exposed group displayed decreased p-NF-κB. Furthermore, mice exposed to GenX or GenX/CBNP displayed increased numbers of BALF macrophages undergoing mitosis and increased Ki67 immunostaining. This was correlated with increased p-STAT3 by western blotting and immunohistochemistry in lung tissue from mice co-exposed to GenX/CBNP. CONCLUSIONS: Pulmonary exposure to GenX suppressed CBNP-induced innate immune response in the lungs of mice yet promoted the proliferation of macrophages and lung epithelial cells.

Carryover effects of chronic exposure to ammonium during the larval stage on post-metamorphic frogs.

Water contamination poses an important challenge to aquatic fauna, including well-documented effects on amphibian larvae. However, little is known about how contamination during the larval stages may affect post-metamorphic phases, or whether resistance may have evolved in some populations. In this work, we tested the hypothesis that chronic exposure to ammonium (a common contaminant in agroecosystems with confirmed effects on anuran tadpoles) during the larval stage of Pelophylax perezi frogs would affect growth and locomotor performance of metamorph, juvenile, subadult and adult stages. We also predicted that the effects of ammonium would be milder in offspring originated from parental agroecosystem frogs than those originating from forests. We compared tadpoles from both habitats either reared in untreated water or chronically exposed to ammonium. We found that exposure to ammonium during the larval stage inflicted effects on morphology (different measures of body size) and swimming speed after metamorphosis until adulthood. However, these effects were not always consistent through post-metamorphic stages and the effects differed as a function of treatment and habitat. In adults, body size and condition were greater in non-ammonium and ammonium exposed individuals, respectively. These differences were not detectable in metamorphs, for which only ammonium-exposed individuals from agroecosystem showed reduced body size in intermediate post-metamorphic stages. In turn, treatment reduced jumping distance only in agroecosystem adults, subadults and juveniles, which was opposite to the trend observed just after metamorphosis. These changes of patterns throughout the ontogeny of P. perezi could be due to processes such as compensatory growth, delayed energy costs derived from it, or early sexual differences that could be present even before they can be accounted for. In summary, this study suggests that exposure to ammonium during larval stages can result in diverse biological and long-term outcomes in later life stages.

Nitrate transporter NRT1.1 and anion channel SLAH3 form a functional unit to regulate nitrate-dependent alleviation of ammonium toxicity.

Ammonium (NH4 + ) and nitrate (NO3 - ) are major inorganic nitrogen (N) sources for plants. When serving as the sole or dominant N supply, NH4 + often causes root inhibition and shoot chlorosis in plants, known as ammonium toxicity. NO3 - usually causes no toxicity and can mitigate ammonium toxicity even at low concentrations, referred to as nitrate-dependent alleviation of ammonium toxicity. Our previous studies indicated a NO3 - efflux channel SLAH3 is involved in this process. However, whether additional components contribute to NO3 - -mediated NH4 + detoxification is unknown. Previously, mutations in NO3 - transporter NRT1.1 were shown to cause enhanced resistance to high concentrations of NH4 + . Whereas, in this study, we found when the high-NH4 + medium was supplemented with low concentrations of NO3 - , nrt1.1 mutant plants showed hyper-sensitive phenotype instead. Furthermore, mutation in NRT1.1 caused enhanced medium acidification under high-NH4 + /low-NO3 - condition, suggesting NRT1.1 regulates ammonium toxicity by facilitating H+ uptake. Moreover, NRT1.1 was shown to interact with SLAH3 to form a transporter-channel complex. Interestingly, SLAH3 appeared to affect NO3 - influx while NRT1.1 influenced NO3 - efflux, suggesting NRT1.1 and SLAH3 regulate each other at protein and/or gene expression levels. Our study thus revealed NRT1.1 and SLAH3 form a functional unit to regulate nitrate-dependent alleviation of ammonium toxicity through regulating NO3 - transport and balancing rhizosphere acidification.

Root GS and NADH-GDH Play Important Roles in Enhancing the Ammonium Tolerance in Three Bedding Plants.

Ammonium is a paradoxical nutrient because it is more metabolically efficient than nitrate, but also causes plant stresses in excess, i.e., ammonium toxicity. Current knowledge indicates that ammonium tolerance is species-specific and related to the ammonium assimilation enzyme activities. However, the mechanisms underlying the ammonium tolerance in bedding plants remain to be elucidated. The study described herein explores the primary traits contributing to the ammonium tolerance in three bedding plants. Three NH4+:NO3- ratios (0:100, 50:50, 100:0) were supplied to salvia, petunia, and ageratum. We determined that they possessed distinct ammonium tolerances: salvia and petunia were, respectively, extremely sensitive and moderately sensitive to high NH4+ concentrations, whereas ageratum was tolerant to NH4+, as characterized by the responses of the shoot and root growth, photosynthetic capacity, and nitrogen (amino acid and soluble protein)-carbohydrate (starch) distributions. An analysis of the major nitrogen assimilation enzymes showed that the root GS (glutamine synthetase) and NADH-GDH (glutamate dehydrogenase) activities in ageratum exhibited a dose-response relationship (reinforced by 25.24% and 6.64%, respectively) as the NH4+ level was raised from 50% to 100%; but both enzyme activities were significantly diminished in salvia. Besides, negligible changes of GS activities monitored in leaves revealed that only the root GS and NADH-GDH underpin the ammonium tolerances of the three bedding plants.

Transcriptomic analysis dissects the regulatory strategy of toxic cyanobacterium Microcystis aeruginosa under differential nitrogen forms.

The critical role of nitrogen in the global proliferation of cyanobacterial blooms is arousing increasing attention. However, the mechanism underlying the algal responses to differential nitrogen forms remains unclarified. The physiological and transcriptomic changes of Microcystis aeruginosa supplied with different nitrogen forms (nitrate and ammonium) were highlighted in this study. The results indicated that ammonium behaves better in stimulating the initial growth in N-limited cells than nitrate. However, a concomitant side effect is that cellular growth and photosynthesis decreased due to photosystem II damage induced by excess absorbed light energy under 10 mg L-1 ammonium. By contrast, adequate nitrate supply favored more efficient photosynthesis, higher biomass yield and microcystin quotas than ammonium. Depending on the supplied nitrogen form, different transcriptomic patterns were observed in M. aeruginosa. Under nitrate, the upregulation of genes involved in Arg biosynthesis, ornithine-urea cycle and photosynthesis increased nitrogen storage and cellular growth, while genes involved in cyclic electron flow around photosystem I and CO2-concentrating mechanism were heightened to dissipate excess energy under high ammonium. These insights provided important clues for understanding the physiological and molecular effects of available nitrogen forms on the frequent outbreaks of cyanobacteria.

Evidence that synergism between potassium and nitrate enhances the alleviation of ammonium toxicity in rice seedling roots.

Ammonium toxicity in plants is considered a global phenomenon, but the primary mechanisms remain poorly characterized. Here, we show that although the addition of potassium or nitrate partially alleviated the inhibition of rice seedling root growth caused by ammonium toxicity, the combination of potassium and nitrate clearly improved the alleviation, probably via some synergistic mechanisms. The combined treatment with potassium and nitrate led to significantly improved alleviation effects on root biomass, root length, and embryonic crown root number. The aberrant cell morphology and the rhizosphere acidification level caused by ammonium toxicity, recovered only by the combined treatment. RNA sequencing analysis and weighted gene correlation network analysis (WGCNA) revealed that the transcriptional response generated from the combined treatment involved cellulose synthesis, auxin, and gibberellin metabolism. Our results point out that potassium and nitrate combined treatment effectively promotes cell wall formation in rice, and thus, effectively alleviates ammonium toxicity.

Comparative studies on the response of Zostera marina leaves and roots to ammonium stress and effects on nitrogen metabolism.

Coastal eutrophication has resulted in the rapid loss and deterioration of seagrass beds worldwide. The high concentration of ammonium in eutrophic aquatic environments has been invoked as the main cause. In this study, leaves and roots of the seagrass Zostera marina were treated with simulated eutrophic seawater with elevated ammonium concentrations. The tolerance to ammonium stress and mechanism of nitrogen metabolism detoxification in different tissues were investigated. The results showed that high ammonium stress significantly affected the growth of leaves and had a negative effect on photosynthesis. The root activity of Z. marina was not inhibited at ammonium concentrations of ≤100 mg/L, indicating that the roots exhibited tolerance to ammonium stress. Increasing ammonium concentrations led to a higher increase of ammonium and free amino acid (FAA) contents in leaves than in roots. However, nitrogen storage decreased in Z. marina leaves after high ammonium treatments. The enzyme activity and gene expression of glutamine synthetase (GS) in roots were significantly higher than in the leaves even under ammonium stress. Meanwhile, ammonium stress increased the enzyme activities and gene expression of glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH) in roots, which suggested that the roots had a strong ability to assimilate ammonium under ammonium stress. In contrast, although the GOGAT and GDH activity and gene expression in the leaves were initially increased, they significantly decreased when the ammonium concentration exceeded 100 mg/L. These results indicated that the concentration of 100 mg/L might be a threshold marking a transition from tolerance to toxicity for the leaves. Our study demonstrates that Z. marina leaves could be prone to higher damage than roots because the mechanism of ammonium assimilation in leaves is more susceptible to ammonium toxicity.

Excessive ammonium assimilation by plastidic glutamine synthetase causes ammonium toxicity in Arabidopsis thaliana.

Plants use nitrate, ammonium, and organic nitrogen in the soil as nitrogen sources. Since the elevated CO2 environment predicted for the near future will reduce nitrate utilization by C3 species, ammonium is attracting great interest. However, abundant ammonium nutrition impairs growth, i.e., ammonium toxicity, the primary cause of which remains to be determined. Here, we show that ammonium assimilation by GLUTAMINE SYNTHETASE 2 (GLN2) localized in the plastid rather than ammonium accumulation is a primary cause for toxicity, which challenges the textbook knowledge. With exposure to toxic levels of ammonium, the shoot GLN2 reaction produced an abundance of protons within cells, thereby elevating shoot acidity and stimulating expression of acidic stress-responsive genes. Application of an alkaline ammonia solution to the ammonium medium efficiently alleviated the ammonium toxicity with a concomitant reduction in shoot acidity. Consequently, we conclude that a primary cause of ammonium toxicity is acidic stress.

Physiologic, metabolomic, and genomic investigations reveal distinct glutamine and mannose metabolism responses to ammonium toxicity in allotetraploid rapeseed genotypes.

Ammonium (NH4+) toxicity has become a serious ecological and agricultural issue owing to increasing soil nitrogen inputs and atmospheric nitrogen deposition. There is accumulating evidence for the mechanisms underlying NH4+-tolerance in rice and Arabidopsis, but similar knowledge for dryland crops is currently limited. We investigated the responses of a natural population of allotetraploid rapeseed to NH4+ and nitrate (NO3-) and screened one NH4+-tolerant genotype (T5) and one NH4+-sensitive genotype (S211). Determination of the shoot and root NH4+ concentrations showed that levels were higher in S211 than in T5. 15NH4+ uptake assays, glutamine synthetase (GS) activity quantification, and relative gene transcriptional analysis indicated that the significantly higher GS activity observed in T5 roots than that in S211 was the main reason for its NH4+-tolerance. In-depth metabolomic analysis verified that Gln metabolism plays an important role in rapeseed NH4+-tolerance. Furthermore, adaptive changes in carbon metabolism were much more active in T5 shoots than in S211. Interestingly, we found that N-glycosylation pathway was significantly induced by NH4+, especially the mannose metabolism, which concentration was 2.75-fold higher in T5 shoots than in S211 with NH4+ treatment, indicating that mannose may be a metabolomic marker which also confers physiological adaptations for NH4+ tolerance in rapeseed. The corresponding amino acid and soluble sugar concentrations and gene expression in T5 and S211 were consistent with these results. Genomic sequencing identified variations in the GLN (encoding GS) and GMP1 (encoding the enzyme that provides GDP-mannose) gene families between the T5 and S211 lines. These genes will be utilized as candidate genes for future investigations of the molecular mechanisms underlying NH4+ tolerance in rapeseed.